Papermaking machines for removing moisture from the source material of paper are generally equipped with a wire part, a press part, and a drier part. These parts are disposed in the order of the wire, press, and drier parts along the direction in which a wet web is conveyed.
In one type of papermaking machine, the wet web is passed in an open-draw. In the press part of this open-draw papermaking machine, there is a location where the wet web is not supported by a paper-making tool, such as a felt or a belt, or a roll, i.e., a location where the wet web passes alone. Problems, such a breakage of the paper, tend occur in this location. The risk of such difficulties increases when such open-draw papermaking machines are operated at high speed. Accordingly, open-draw papermaking machines have been limited with respect to higher-speed operation to some extent.
In view of the foregoing difficulties, in recent years, a type of papermaking machine in which a wet web is passed in a closed-draw has become mainstream. In the press part of such closed-draw papermaking machines, the wet web is conveyed while being placed on a felt for papermaking or a belt for transferring a wet web. As a result, there is no location where the wet web passes alone, as in open-draw papermaking machines. As a result, it is possible to operate such papermaking machines at higher speeds and stabilization of operation also results.
An example of a press part of a closed-draw papermaking machine is described in detail with reference to FIG. 3. A papermaking part of the closed-draw papermaking machine includes a wire part, a press part and a drier part, in sequence in the direction in which a wet web is conveyed. The press part in FIG. 3 is an example of a press part for a closed-draw papermaking machine having two press devices. A first press device 1P formed of a top roll 1a and a bottom roll 1b and a second press device 2P formed of a top roll 2a and a bottom roll 2b. The first press device 1P and the second press device 2P are provided adjacently in series along the direction in which the wet web is conveyed.
In the closed-draw papermaking machine shown in FIG. 3, a plurality of felts PUF, 1PBF, and 2PTF for papermaking are used. The pickup felt PUF receives the wet web WW from the wire part WF and is used in the top roll side of the first press device 1P. The bottom felt 1PBF receives the wet web WW from the pickup felt PUF and is used in the bottom roll side of the first press device 1P. The top felt 2PTF receives the wet web WW from the bottom felt 1PBF and is used in the top roll side of the second press device 2P. In addition, a belt TFB for transferring a wet web receives the wet web WW from the top felt 2PTF and is used in the bottom roll side of the second press device 2P. The belt TFB for transferring a wet web may also be used in the top roll side of the second press device 2P; however, in this case, the felt for papermaking is used in the bottom roll side of the second press device 2P from the viewpoint of squeezing out water from the wet web.
In these felts PUF, 1PBF, and 2PTF for papermaking, batt fibers are needled on one surface or both surfaces of a reinforcing fiber substrate, and the belt TFB for transferring a wet web is an endless belt, in which a polymer resin is layered on at least the wet web contact surface of the reinforcing fiber substrate. The felts PUF, 1PBF, and 2PTF and the belt TFB are supported by guide rolls GR and suction rolls SR, as illustrated in FIG. 3.
In an alternative papermaking machine configuration, a shoe press device is formed in any one or more of the top rolls 1a, 2a and bottom rolls 1b, 2b in the press device. In such a papermaking machine in which the shoe press device is formed in the press device, a felt for papermaking is used on a shoe press device side and a belt for transferring a wet web is used on a roll side facing the shoe press device. The number of press devices disposed in the press part is not limited to two exhibited in one example as described above and one press device or three or more press devices may also be disposed.
In the example of FIG. 3, water is squeezed from the wet web WW by the first press device 1P and the second press device 2P. When attention is focused on the second press device 2P, the top felt 2PTF which is used in the second press device 2P is water-permeable whereas the belt TFB for transferring a wet web is not water-permeable. Thus, in the second press device 2P, moisture moves from the wet web WW to the top felt 2PTF and is drained to the outside of the press device system but some moisture remains in the top felt 2PTF.
Immediately after leaving the press part of the second press device 2P, the respective volumes of the top felt 2PTF, the belt TFB for transferring a wet web, and the wet web WW pinched between them expand because of a sudden release of pressure. Due to this expansion and the capillary phenomenon of the pulp fibers constituting the wet web WW, a re-wetting phenomenon occurs during which some of the moisture remaining in the top felt 2PTF moves to the wet web WW.
Nevertheless, since the belt TFB for transferring a wet web is not water-permeable as described above, moisture is not retained in the polymer resin of the wet web contact surface. Thus, the re-wetting phenomenon from the belt TFB for transferring a wet web rarely occurs; and the belt TFB for transferring a wet web contributes to improvement in efficiency of squeezing out water of the wet web WW. The wet web WW leaving the press part 2P is conveyed by the belt TFB for transferring a wet web, moves to a drier fabric DF via the suction roll SR, and is conveyed to the drier part.
In the papermaking step, the belt for transferring a wet web preferably contributes to:
1) adhesiveness and releasability of the wet web on the wet web-side surface of the belt for transferring a wet web;
2) cooperation with the felt for papermaking in the press device; and
3) stable traveling performance in the closed-draw and durability.
Various belts for transferring a wet web have conventionally been proposed to perform the above-described functions.
For example, U.S. Pat. No. 7,722,741 discloses a belt for transferring a wet web formed of an impermeable polymer layer having a wet web contact side and a roll side, wherein the surface structure of the roll side has a porous structure or a surface roughness Ra (arithmetic mean roughness) of 3-40 μm. In the above-described surface structure, grooves and protrusions are formed so that the porous structure is maintained under pressurization by a press device. Receptivity for fluid acting on the roll side is consequently formed to prevent skids between a belt for transferring a wet web and various rolls, particularly a guide roll for controlling the travel position of the belt for transferring a wet web, caused by a hydroplaning phenomenon to maintain stable traveling performance. Thus, the belt for transferring a wet web is further prevented from being damaged by fluid under pressurization.
U.S. Pat. No. 7,776,188 discloses a belt for transferring a wet web formed of an impermeable polymeric layer having a wet web contact side and a roll side, wherein a plurality of grooves or flute-like recesses are formed in the roll side and the respective surface roughnesses of the plurality of recesses are lower than the surface roughness of the roll side surface. The belt for transferring a wet web is provided with static friction crucial for operating the belt for transferring a wet web by the roll side surface having surface roughness to some extent. A hydroplaning phenomenon is prevented by further lowering the surface roughness of each of the plurality of recesses, and fluid or impurities entering into the recesses are more effectively shaken from the belt.
However, the belts for transferring a wet web described in U.S. Pat. No. 7,722,741 and U.S. Pat. No. 7,776,188, have had problems, such as cracks caused by forming the grooves in the roll side and groove marks copied on the wet web via the wet web contact side surface of the belt for transferring a wet web. In addition, in the belt for transferring a wet web described in U.S. Pat. No. 7,722,741, although receptivity for fluid is formed at the surface roughness of the surface structure of the roll side of 3-40 μm, the belt has been insufficient from the viewpoint of preventing a hydroplaning phenomenon or damage to the belt for transferring a wet web.
WO 2008/131979 discloses a belt for transferring a wet web formed of an impermeable polymeric layer having a wet web contact side and a roll side, wherein the wear resistance of the roll side is made to be more than the wear resistance of the wet web side. In the belt for transferring a wet web described in WO 2008/131979, the life of the belt for transferring a wet web is improved by mixing a polymer layer constituting the roll side with fibers or calcium carbonate to improve the wear resistance of the roll side.
JP-A-2000-027088 proposes a belt for a papermaking step. The belt is coated with a polymeric resin and includes a reinforcing substrate (reinforcing fiber substrate). The coating of a polymeric resin material is provided on the front side of the reinforcing substrate, and a staple fiber batt is attached to the back side of the reinforcing substrate. The reinforcing substrate has an endless loop shape and the front and back sides. The front side is the outside of the endless loop, and the back side is the inner side of the endless loop. The staple fiber batt has a smooth molten surface, and there is no fiber end that protrudes from the staple fiber batt in the surface. The belt for a papermaking step is a belt for transferring a wet web, including a water-impermeable coating resin layer placed on the wet web contact side of the reinforcing substrate and a batt fiber layer placed on the roll side of the reinforcing substrate. The fibers on the roll side surface of the batt fiber layer are molten to smooth the surface. In this belt for transferring a wet web, since the roll side surface is smooth, impurities rarely adhere to the belt and damage to the batt fiber layer or the reinforcing substrate due to the adhesion of impurities is prevented.
However, the belts for transferring a wet web described in WO 2008/131979 and JP-A-2000-027088 are insufficient from the viewpoint of preventing a hydroplaning phenomenon since the roll sides are smooth. Further, since the roll side is constituted by the batt fiber layer and a molten material thereof, the belt for transferring a wet web as described in JP-A-2000-027088 has comparatively lower strength than that of the polymeric resin coat layer and is easy to be damaged by high-pressure washing applied to the roll side surface in the course of or during use. As a result, the fibers of the batt fiber layer may fall off with the damage to shorten the life of the belt for transferring a wet web. In other words, it has been necessary to control the feed rate (guiding characteristic) of the belt for transferring a wet web to 1,300 m/min or less.